Issue #118 - document security.NTACL for samba-dc

[instantlinux]

Summary of Changes

README for samba-dc is updated to point at the rather vague documentation provided for security.NTACL defined in version 4.18 of the samba-dc image.

Why is this change being made?

By user request.

How was this tested? How can the reviewer verify your testing?

Completion checklist

• [ ] The pull request is linked to all related issues
• [ ] This change has unit test coverage
• [x] Documentation has been updated
• [ ] Dependencies have been updated and verified